The master developmental regulator Jab1/Cops5/Csn5 is essential for proper bone growth and survival in mice

Jab1, also known as Csn5/Cops5, is a key subunit of the COP9 Signalosome, a highly conserved macromolecular complex. We previously reported that the conditional knockout of Jab1 in mouse limb buds and chondrocytes results in severely shortened limbs and neonatal lethal chondrodysplasia, respectively. In this study, we further investigated the specific role of Jab1 in osteoblast differentiation and postnatal bone growth by characterizing a novel mouse model, the Osx-cre; Jab1(flox/flox) conditional knockout (Jab1 cKO) mouse, in which Jab1 is deleted in osteoblast precursor cells. Jab1 cKO mutant mice appeared normal at birth, but developed progressive dwarfism. Inevitably, all mutant mice died prior to weaning age. The histological and micro-computed tomography analysis of mutant long bones revealed severely altered bone microarchitecture, with a significant reduction in trabecular thickness. Moreover, Jab1 cKO mouse tibiae had a drastic decrease in mineralization near the epiphyseal growth plates, and Jab1 cKO mice also developed spontaneous fractures near the tibiofibular junction. Additionally, our cell culture studies demonstrated that Jab1 deletion in osteoblast precursors led to decreased mineralization and a reduced response to TGF beta and BMP signaling. Moreover, an unbiased reporter screen also identified decreased TGF beta activity in Jab1-knockdown osteoblasts. Thus, Jab1 is necessary for proper osteoblast differentiation and postnatal bone growth, likely in part through its positive regulation of the TGF beta and BMP signaling pathways in osteoblast progenitor cells.

Automated summary

This study investigated the specific role of Jab1 in osteoblast differentiation and postnatal bone growth using a novel mouse model. The results showed that Jab1 is essential for proper osteoblast differentiation and postnatal bone growth, likely through its positive regulation of the TGF beta and BMP signaling pathways in osteoblast progenitor cells. Knockout of Jab1 led to progressive dwarfism, altered bone microarchitecture, reduced mineralization, and decreased response to TGF beta and BMP signaling, ultimately resulting in spontaneous fractures and premature death in mutant mice.